Thermal batteries are relatively well known in the art and comprise a plurality of thermal cells each of which includes an anode, a cathode, and an electrolyte and a heat source, usually an ignitable, exothermically reactive chemical charge or pyrotechnic. A variety of electrochemical systems are known for use in thermal cells.
The electrolytes are generally mixtures of alkali metal halides, most commonly a eutectic mixture of LiCl and KCl. The cathodes, generally referred to as depolarizers, comprise materials that are reduced in the electochemical cell reaction and include phosphates, metal oxides, borates and chromates. The most widely used depolarizer material is either calcium chromate or vanadium pentoxide. It is present practice to mix the electrolyte and depolarizer with a binder in powder form and press the mixture into a wafer such as set forth in U.S. Pat. Nos. 3,677,822; 3,425,872; and 3,527,615.
The most commonly used anode material is calcium, typically in the form of a coating on a nickel or iron current collector. Other materials such as magnesium and solid lithium alloys have been used. See U.S. Pat. No. 3,367,800.
Conventional thermal cells such as calcium/lithium chloride, potassium chlorides/calcium chromate experience discharge reactions in which the cell components react chemically rather than electrochemically with no electrical power generation. Such reactions result in the rapid deterioration of the cell at operating temperatures which temperatures are of a relatively narrow range. Exceeding this narrow range, of about 100.degree. C., results in a self-discharge reaction that further heats the cell to accelerate the self-discharge creating a thermal runaway. To overcome the problem of thermal runaway, deteriorization and to provide batteries having higher energy density, higher power density and longer life, anodes comprising a foraminous inert metal substrate wettable by and filled with an electrochemically active anode metal, melting at temperatures below cell operating temperatures, were provided. U.S. Pat. Nos. 3,891,460 and 3,930,888. The anodes taught and disclosed in these patents generally comprise a housing having an impervious metal portion in electrical contact with the active anode metal, typically lithium, and a porous refractory fiber portion in sealing engagement against the periphery of the metal portion. Batteries utilizing these anodes comprise a stack of cells in recurring sequence of an anode, a wafer containing electrolyte, depolarizer and binder, and a combustible composition that serves as a heat source and a cathode current collector. These batteries are activated by heating to a temperature above the melting point of the anode metal and melting point of the electrolyte.
Batteries utilizing the improved anode structures provide substantial advantages over thermal batteries without such anodes. It is desirable in many applications, however, to have a battery wherein the internal resistance is reduced near the end of its life when a high current pulse is often required. Moreover, lower internal resistance results in improved electronic conductivity with smaller voltage drops under pulsed loads. Furthermore, in numerous applications it is desirable to decrease the start time of a thermal battery.